How to improve D-Noise in blender?
It should not be relative with denoise, without your screenshot, it's hard to tell what's going on your setting.
But a common blur in preview is because the simple viewport option is set to auto:

Set it to 1-px and see if the problem resolved.

And you might want to start denoise your view port after certain sample (20?) instead of denoising from beginning (which result in a laggy viewport been denoise every sample):